Introduction

Few conditions in neonatal medicine present clinicians with challenges as complex and time-sensitive as hypoxic-ischaemic encephalopathy (HIE). Despite considerable advances in obstetric care, neonatal resuscitation, and intensive care medicine, perinatal asphyxia continues to affect approximately one to three infants per 1,000 live births in high-income countries, with substantially higher rates reported in low- and middle-income settings. Survivors frequently face lifelong neurodevelopmental consequences including cerebral palsy, epilepsy, cognitive impairment, behavioural disorders, and sensory deficits.

The introduction of therapeutic hypothermia transformed neonatal neurocritical care and remains the only intervention with proven neuroprotective efficacy for moderate and severe HIE. Nevertheless, nearly half of treated infants either die or develop significant neurological impairment despite receiving optimal cooling therapy. Such observations suggest that brain injury following perinatal asphyxia cannot be fully explained by cerebral mechanisms alone.

For decades, clinical attention in HIE has focused primarily on neuronal energy failure, excitotoxicity, oxidative stress, and neuroinflammation. Although these pathways undoubtedly play central roles, severe intrapartum hypoxia rarely affects the brain in isolation. The fetus responds to oxygen deprivation through redistribution of cardiac output toward vital organs, a compensatory mechanism commonly referred to as the "brain-sparing effect." When hypoxia becomes prolonged or severe, however, these adaptive responses become overwhelmed, resulting in widespread systemic injury involving the myocardium, kidneys, lungs, liver, and gastrointestinal tract.

Among these organs, the heart and kidneys appear to exert particularly important influences on neurological recovery. Myocardial dysfunction may compromise cerebral perfusion during the vulnerable reperfusion period, while acute kidney injury alters fluid homeostasis, drug clearance, inflammatory signaling, and metabolic stability. Conversely, evolving cerebral injury can influence autonomic regulation and cardiovascular function, creating a dynamic cycle of reciprocal organ interactions.

The concept of a cardio-renal syndrome is well established in adult medicine, particularly in patients with heart failure and critical illness. In neonatology, however, a broader and more integrated understanding of the relationship between cardiac function, renal physiology, and cerebral perfusion has only recently begun to emerge. Increasing use of targeted neonatal echocardiography, near-infrared spectroscopy, continuous electroencephalographic monitoring, and novel biomarkers has highlighted the existence of a complex cardio-renal-brain axis that may significantly influence outcomes following hypoxic-ischaemic injury.

Recognition of this interconnected physiology has important clinical implications. Management strategies based solely on blood pressure thresholds or neurological examination may fail to identify occult circulatory compromise, impaired cerebral autoregulation, or evolving renal dysfunction. Precision neurocritical care therefore requires a shift from organ-specific treatment toward integrated haemodynamic assessment and individualized therapeutic decision-making.

This review explores current evidence regarding cardiovascular dysfunction, acute kidney injury, and cerebral autoregulatory disturbances in neonatal HIE, examines the physiological interactions between these systems during therapeutic hypothermia, and discusses emerging approaches that may support the development of precision haemodynamic management in neonatal neurocritical care.

Evolution of the Brain-Centric Model of HIE and the Emergence of the Cardio-Renal-Brain Axis

From a Neurological Disease to a Multisystem Disorder

For many years, neonatal hypoxic-ischaemic encephalopathy was regarded primarily as a disease of cerebral injury. Research efforts understandably focused on neuronal apoptosis, excitotoxicity, oxidative stress, mitochondrial dysfunction, and inflammatory cascades occurring within the central nervous system. This brain-centred approach led to major advances in understanding the mechanisms of primary and secondary energy failure and ultimately paved the way for the introduction of therapeutic hypothermia.

However, clinical observations have increasingly challenged the concept of HIE as an isolated neurological disorder. Infants with severe perinatal asphyxia frequently demonstrate evidence of myocardial dysfunction, pulmonary hypertension, acute kidney injury, hepatic impairment, coagulopathy, adrenal insufficiency, and gastrointestinal injury within the first hours after birth. Indeed, the severity of extracerebral organ dysfunction often parallels the degree of neurological injury and may independently influence survival and neurodevelopmental outcome.

Recent studies suggest that approximately 60–80% of infants with moderate or severe HIE exhibit some degree of cardiovascular compromise, while acute kidney injury develops in up to 50% of cooled infants. These findings indicate that systemic hypoxia-ischaemia produces a complex pattern of organ crosstalk rather than isolated cerebral injury.

The Fetal Brain-Sparing Response

During acute hypoxia, the fetus activates several adaptive mechanisms aimed at preserving oxygen delivery to vital organs. Redistribution of cardiac output occurs through peripheral vasoconstriction and preferential perfusion of the brain, heart, and adrenal glands, a physiological response commonly referred to as the brain-sparing effect.

This adaptive circulation is mediated through:

  • sympathetic nervous system activation,
  • catecholamine release,
  • peripheral vasoconstriction,
  • increased coronary blood flow,
  • cerebral vasodilatation.

Under mild or transient hypoxic conditions, these compensatory mechanisms may successfully preserve cerebral oxygenation and limit organ injury.

However, prolonged or severe asphyxia overwhelms these protective responses.

Progressive myocardial hypoxia leads to declining cardiac output, impaired ventricular contractility, and loss of compensatory redistribution. Once systemic perfusion falls below a critical threshold, oxygen delivery to both central and peripheral organs deteriorates rapidly, initiating widespread cellular injury.

Reperfusion Injury: The Beginning of Organ Crosstalk

The restoration of circulation following successful resuscitation is essential for survival but paradoxically initiates additional injury pathways.

Reoxygenation triggers:

  • generation of reactive oxygen species,
  • mitochondrial dysfunction,
  • endothelial activation,
  • inflammatory cytokine release,
  • leukocyte recruitment,
  • microvascular dysfunction.

This phenomenon, commonly referred to as reperfusion injury, contributes substantially to secondary energy failure occurring during the first 6–48 hours after birth.

Importantly, these processes occur simultaneously within multiple organs.

Myocardial injury reduces systemic blood flow and oxygen delivery, renal dysfunction impairs metabolic homeostasis and inflammatory clearance, while cerebral autoregulatory disturbances increase susceptibility to fluctuations in perfusion pressure. Rather than acting independently, these injuries amplify one another through a network of haemodynamic and inflammatory interactions.

The Heart as a Driver of Neurological Recovery

The neonatal myocardium possesses limited contractile reserve compared with older children and adults. Reduced compliance, immature calcium handling, and limited ability to augment stroke volume make the newborn heart particularly vulnerable to hypoxic injury.

Following perinatal asphyxia, myocardial dysfunction may manifest as:

  • impaired left ventricular contractility,
  • right ventricular dysfunction,
  • reduced cardiac output,
  • tricuspid regurgitation,
  • systemic hypotension,
  • persistent pulmonary hypertension of the newborn (PPHN).

Importantly, normal blood pressure does not necessarily indicate adequate systemic blood flow.

Several studies using functional echocardiography have demonstrated that infants with HIE may exhibit significantly reduced cardiac output despite maintaining blood pressure values within conventional reference ranges. Such occult low-output states may contribute to impaired cerebral perfusion during the critical period of secondary brain injury.

These observations have shifted attention away from isolated blood pressure targets toward more comprehensive haemodynamic assessment.

The Kidney as an Active Participant Rather Than a Passive Victim

Historically, acute kidney injury in HIE was viewed primarily as a marker of severe asphyxia rather than an active contributor to disease progression.

Emerging evidence suggests otherwise.

Renal dysfunction may contribute to neurological injury through several mechanisms:

  • impaired fluid regulation,
  • electrolyte disturbances,
  • altered clearance of anticonvulsants and vasoactive medications,
  • amplification of systemic inflammation,
  • accumulation of metabolic toxins,
  • endothelial dysfunction.

Experimental studies have demonstrated bidirectional communication between injured kidneys and the central nervous system mediated through inflammatory cytokines, oxidative stress pathways, and neurohumoral activation.

The kidney therefore functions not merely as an innocent bystander but as an active component of the evolving neurocritical illness.

The Cardio-Renal-Brain Axis: A New Conceptual Framework

The traditional organ-based model of neonatal intensive care may no longer adequately reflect the complexity of modern neurocritical care.

A more integrated model recognizes that:

  • cardiac dysfunction influences cerebral perfusion,
  • cerebral injury alters autonomic cardiovascular regulation,
  • renal dysfunction modifies inflammatory and metabolic responses,
  • therapeutic interventions targeting one organ may affect another.

The outcome following HIE therefore reflects not only the severity of cerebral injury but also the resilience of the entire physiological network supporting cerebral recovery.

This concept forms the basis of the neonatal cardio-renal-brain axis, an emerging framework that may help guide future strategies in individualized haemodynamic management.